1. Field of the Invention
The present invention relates to scanning optical systems that scan a surface to be scanned by directing a beam of light thereto from a light source through a deflector and a lens system. More particularly, the present invention is directed to a scanning optical system having a correcting function that is designed to detect a displacement from the focus of an image forming spot of a beam of light which is on the surface to be scanned, the displacement being attributable to changes in ambient conditions such as temperature.
2. Discussion of the Related Art
Recording apparatuses using scanning optical systems are widely used in recent years. Examples are laser beam printers, which modulate a laser beam source in accordance with an image signal, cyclically deflect a laser beam from the modulated laser beam source with a deflector, converge the laser beam in spot form on a photosensitive recording medium with a lens system, and make a recording of an image by exposing the image through scanning.
In such conventional laser beam printers, a problem has been encountered. That is, changes in ambient temperature cause thermal deformation of components constituting the lens system, and as a result, the laser beam converging position on the photosensitive body is displaced, thus causing impairment of image quality.
To overcome this problem, e.g., Japanese Patent Unexamined Publication No. Hei 2-73212 discloses a system in which focus adjustment is made by disposing a plurality of detectors, each having an opening in a direction parallel with a scanning direction, at positions optically equivalent to a surface to be scanned and by comparing or calculating signals from detecting sections thereof.
However, the above-mentioned conventional system uses a plurality of light beam detectors, and this construction presents the problem of requiring not only a large installation space but also a large cost.